The Expires header specifies when content will expire,or how long content to be fresh. After the time expired, the site will always check back with the server to see whether the database has been changed or not.
Expires: -1 The Expires header specifies when the content should be considered to be out of date. The value -1 indicates that the content expires immediately and would have to be re-requested before being displayed again.
Note: Never use Expires = 0 to prevent caching in your site. The Expires header is sent by the server and passed through to browser by the Server. Expires=0 header can mistakenly return cached content. To solve this problem, set the Expires header to a fixed date that is definitely in the past.
In JSP, setting caching to forever using the Expires header is as simple as using the code that follows:
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